Sterilization by the application of boiling water or steam to the article to be sterilized has been carried out for many years. More recently there has arisen in certain fields, such as in medicine and in space exploration, the need to employ a different sterilant because certain articles used in these fields cannot withstand the temperatures or the moisture associated with steam sterilization.
One sterilant that has become widely used is ethylene oxide because not only is it an effective sterilant but also its residues volatize relatively quickly from the article sterilized. Although ethylene oxide may be used by itself to carry out the sterilization, this is generally not done because ethylene oxide is highly flammable. Instead ethylene oxide sterilant is generally used in a mixture with a flame retardant. The flame retardant, however, must complement the properties of the ethylene oxide or the beneficial effects of the ethylene oxide will be lost. Over the last two decades the flame retardant of choice for use with ethylene oxide in a sterilant mixture has been dichlorodifluoromethane, known in the industry as CFC 12. The most commonly used sterilant mixture comprises 27.3 mole percent (12 weight percent) ethylene oxide and 72.7 mole percent (88 weight percent) CFC 12. This mixture is commonly referred to in the industry as 12-88.
Recently a problem has arisen in the use of CFC 12 because it is one of the chlorofluorocarbons believed to cause significant damage to the ozone layer in the upper atmosphere. Accordingly worldwide reduction and elimination of the use of CFC 12 is now underway. This has created a problem for the use of ethylene oxide as a sterilant.
As mentioned above, ethylene oxide may be used by itself as a sterilant. However the explosion danger of such use makes it acceptable for only a relatively few applications at locations which have experienced and sophisticated handlers available at all times.
One flame retardant which is known for use with ethylene oxide is carbon dioxide. However because of the characteristics of carbon dioxide, a nonflammable ethylene oxide carbon dioxide mixture contains less than 40 percent of the ethylene oxide per unit volume as does 12-88. Thus, sterilization must be carried out either at higher pressures or for longer contact times. Furthermore the large difference in the vapor pressures of ethylene oxide and carbon dioxide causes the mixture to separate upon withdrawal from the storage tank or cylinder, raising the danger of delivering a sterilant mixture rich in carbon dioxide, which won't sterilize, or rich in ethylene oxide, which is explosive.
Accordingly it is an object of this invention to provide an improved sterilant mixture employing ethylene oxide which overcomes the deficiencies of the known sterilants.
It is another object of this invention to provide an improved sterilization method using a sterilant mixture employing ethylene oxide which overcomes the deficiencies of the known sterilization methods.